1. Field of the Invention
The present invention relates to a rare earth phosphor for use in a cathode-ray tube (CRT), and more particularly to a rare earth phosphor for use in a CRT for a liquid-crystal-color shutter display system.
2. Description of the Related Art
Recently a CRT for use in a liquid-crystal-color-shutter (hereinafter referred to as "LCS") display system has been drawing attention. I the LCS display system, the LCS is placed in front of a monochrome CRT. The polarizer and .pi. cells of the LCS separate emission from the monochrome CRT into the blue(B), green(G) and red(R) components, and then these components are made to overlap appropriately another or two others and the brightness of the CRT is appropriately changed, thereby generating various kinds of colors.
In a shadow-mask-type CRT, a variety of colors is produced by applying three electron beams from three electron guns to three phosphor dots of G, B and R. However, in the monochrome CRT, one electron gun can serve a variety of colors. The LCS display system using the monochrome CRT is therefore advantageous in providing a good resolution. In addition, since the LCS is placed in front of the monochrome CRT, the color of the display surface appears black, not gray as in the shadow-mask-type CRT. The LCS display system is advantageous in providing excellent contrast, as well.
However, the LCS has but a low transmittance. To obtain the brightness to the same extent as in a conventional CRT, it is necessary to increase the brightness of the monochrome CRT placed behind the LCS.
It is demanded that phosphors used for a monochrome CRT in a LCS display system should have a single phosphor capable of emitting white-like luminescence. More preferably, a phosphor having a luminescence spectrum comprising three peaks in ranges of wavelengths of G, B, and R, respectively. When a phosphor mixture is used for a monochrome CRT, and an uneven color is displayed depending on a size of a spot on which an electron beam impinges. Furthermore, for a phosphor used in the monochrome CRT, the ratio of three peaks of the phosphor must be adjusted to a value suitable for the polarizer used known as an example of a phosphor satisfying the above-described requirements is P45 phosphor (JEDEC) represented by (Y, Tb).sub.2 O.sub.2 S. Another example that may be used is (Y, Tb, Sm).sub.2 O.sub.2 S phosphor disclosed in Published Examined Japanese Patent Application No. 53-28146.
FIG. 1 shows a luminescence spectrum of (Y.sub.0.9967 Tb.sub.0.0025 Sm.sub.0.0008).sub.2 O.sub.2 S phosphor. In a LCS, for example, peaks in the vicinity of 418 nm, 545 nm, and 608 nm, used as a blue component, a yellow-green component, an orange component--all dispersed from the above described phosphor--are separated, and then made to overlap another or two others, thereby displaying desired individual colors.
As described above, the brightness of a LCS display totally depends on the brightness of a monochrome CRT placed behind the LCS, that is, the brightness of the phosphor. Accordingly, in order to improve the brightness of a LCS, it is necessary to excite the phosphor by increasing the current density. However, since conventional phosphors do not have good current properties, the brightness of the LCS has not been increased. In other words, the conventional phosphors have a tendency to generate so-called "brightness saturation", which is the phenomenon that the brightness can no longer be increased even if a phosphor is excited at a current density beyond a certain value. There is another problem with the conventional phosphors. When a phosphor is kept excited at the high current density, it develops a phenomenon called "burning", which changes color of the phosphor itself, reducing the brightness. Thus, the brightness of a monochrome CRT has not been improved.